Prior to a June, 1984, FCC decision, pay telephones were the exclusive province of local telephone companies. Others were precluded from the business of providing pay telephone services. Today, however, subject to state Public Utility Commission regulations, Customer Owned Coin Operated Telephone ("COCOT") service is permitted. An outgrowth of COCOT service has been the private operation of institutional telephone services.
Calls placed through COCOT equipment are frequently handled by an automated operator service ("AOS"), i.e., call handling equipment and procedures which can automatically route and complete local and long distance calls without the intervention of a live operator. Typically, an AOS has the ability to complete collect calls and to bill users of that service for both intra- and inter-LATA calls. However, the use of an AOS system opens the possibility of fraudulent activity in certain instances.
In many institutions, such as prisons, the phone calls placed by an inmate or patient are restricted to certain previously approved numbers, such as those of family, friends, defense lawyers, etc. To minimize the possibility of fraud or harassment, calls cannot be made to other numbers, such as judges, prosecutors, witnesses, commercial establishments and members of the public at large. Nevertheless, these security measures can be circumvented if an accomplice answering a phone having an approved number uses features of the phone, such as three-way or conference calling, to establish a connection with an unapproved third party telephone. To avoid these problems, it would be desirable if an AOS system utilized at a prison, for example, could be able to detect when the other, i.e., remote party, has commenced a three-way call (i.e., a three-party calling service initiated by activation of the hook switch) or a conference call (such as that resulting from the bridging of two lines). It should do so reliably, in that it should detect as many of the three-way or conference calls as possible, and it should not be subject to "false detects." False detects are at least irritating, if not expensive, for one or more of the participants to the telephone call. In particular, the AOS should not be prone to false detects resulting from certain other events, such as the announcement or activation of call waiting service which may occur during a normal telephone call.
Several methods of detecting and confirming the detection of an attempted three-way call are known in the prior art. Most of these are intended to recognize one or more changes in the electrical signal resulting from a party's flashing the hook switch to activate a three-way calling feature. The flashing results in a temporary disconnect and reconnect, which can be observed as a momentary interruption of loop current in the immediate circuit associated with the telephone at which the hook flash occurred. Since the current is not interrupted further "down the line," this technique is not available to detect a hook flash at a remote telephone, i.e., at the other telephone involved in a two-party call. In addition, it cannot distinguish between a hook flash event used for activation of three-way calling service and other hook flash events.
In addition, a battery polarity reversal may be observed at the telephone company central office when the local party performs a hook flash. However, the polarity reversal is not observable down the line to the other party to the telephone call. Thus, this attribute of a hook-flash cannot be employed as a detection mechanism, except at the local central office.
U.S. Pat. Nos. 5,319,702 and 5,539,812, which are commonly assigned to the assignee of the present invention, describe techniques to recognize certain characteristics of the signal received at a local telephone when a remote party flashes the hook switch. Although these techniques reliably detect the vast majority of hook flash signals performed by a remote party to a telephone call, they are not one hundred percent perfect under all conditions. Coupled with the "window analyzation" techniques described in the '702 Patent, these methods can also reliably differentiate an attempted three-way call from a call waiting signal. On the other hand, the techniques described in these patents do not consistently detect conference calls established by "bridging" two lines or by other techniques, such as PBX switching, which do not utilize flashing of the hook switch. In addition, the technology of the '702 and '812 Patents does not reliably detect whether a remote telephone has been forwarded.
Thus, it is desirable to provide a more infallible way of detecting attempts by a remote party to circumvent telephone security procedures, such as those typically employed with inmate telephone calls.